<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        function backday() {
            var year = prompt("请输入年份")
            if(isleanyear(year)){
                alert('当前年份是闰年2月份有29天')
            }
            else{
                alert('当前年份是平年2月份有28天')
            }
          }
          backday()
          function isleanyear(year){
              var flag = false;
              if(year%4==0&&year%100!=0||year%400==0){
                  flag=true;
              }
              return flag;
          }
    </script>
</body>
</html>